1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gas and a method for production thereof. More particularly, it relates to a catalyst for purifying exhaust gas by simultaneous removal of such harmful components as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x) contained in the exhaust gas from the internal combustion engine of an automobile, for example, especially a catalyst for purifying exhaust gas which retains outstanding durability even when it is used under such harsh conditions as experienced in an oxidizing atmosphere at elevated temperatures and which exhibits a high scrubbing performance at low temperatures to the harmful components.
2. Description of the Prior Art
Heretofore, there have been proposed numerous catalysts for purifying exhaust gas emanating from the internal combustion engine of an automobile, for example. At present, three-way catalysts for simultaneous removal of CO, HC, and NO.sub.x constitute the mainstream of the trend of the catalyst for purifying exhaust gas.
The three-way catalysts mentioned above are generally produced by depositing such active components as platinum (Pt), palladium (Pd), and rhodium (Rh) either singly or jointly as dispersed on such a refractory inorganic oxide as alumina or zirconia in a powdered form thereby preparing a catalyst composition and coating a monolithic carrier of cordierite, for example, with the catalyst composition. Of these three-way catalysts, that which has platinum-family metals including rhodium dispersed densely on activated alumina and also has cerium oxide (ceria incorporated in the catalyst composition is claimed to possess an outstanding purifying ability.
Further, the practice of adding to the catalyst active composition a promoter component selected from among the compounds of rare earth elements, the compounds of the elements of Group VIII, the alkali metal compounds, and the alkaline earth metal compounds for the purpose of promoting and stabilizing the catalytic activity of noble metals as the active component of catalyst is now in vogue. Particularly, cerium oxide (CeO.sub.2), a rare earth compound, manifests a conspicuous effect as a promoter catalyst and has found widespread utility
Generally, rhodium is a noble metal component which contributes in a large measure to the catalyst's ability to purify the waste gas of NO.sub.x and permit combustion at low temperatures. It has been known, however, that under such harsh conditions as experienced in an oxidizing atmosphere at elevated temperatures, rhodium is diffused in an alumina and consequently deprived of activity [refer to J. Catal., 50, 407 (1977)] and rhodium and ceria undergo undesirable interaction. In Japanese Patent Laid-open SHO 61(1986)-11,147, for example, it is disclosed that the interaction of rhodium and alumina is diminished by regulating the particle diameter of rhodium. This method, however, necessitates intricate steps as for wet reduction. We have confirmed that this method is not sufficient for the purpose of curbing the interaction of rhodium and ceria. As ways of incorporating ceria in the catalyst composition, a method which comprises depositing ceria on the catalyst composition by impregnating activated alumina with a solution of ceria and a method effects the incorporation by pouring the bulk of an oxide, hydroxide, or carbonate of cerium into the other components of the catalyst composition have been known. When the ceria is deposited as on activated alumina, however, a disadvantage ensues that the ceria particles are liable to grow and the ceria is deficient in thermal stability. When the bulk of cerium, compound is poured into the catalyst composition, there is a disadvantage that the consumption of ceria is so large as to render the produced catalyst very expensive and the frequency of contact between ceria and rhodium is increased to an extent of inducing an undesirable interaction and consequent loss of activity. Japanese Patent Laid-Open SHO 60(1985)-171,223 and Japanese Patent Laid-Open SHO 62(1987)-168,544 have a disclosure to the effect that all the rare earth elements are as effective in stabilizing zirconium as yttrium, calcium, and magnesia. The effect which such rare earth elements used in a catalyst are to manifest in aiding in the catalysis remains yet to be clarified.
In the field of catalysts, zirconia is used mostly for the purpose of stabilizing the specific surface area and other similar physical properties of the catalyst composition. As concerns the use of zirconia as a carrier for a platinum-family metal, various methods are proposed as in Japanese Patent Publication SHO 57(1982)-29,215 and Japanese Patent Laid-Open SHO 57(1982)-153,737, for example.
Zirconia possesses highly desirable properties as a carrier for a platinum-family metal. It nevertheless is highly susceptible of thermal deterioration and liable to undergo heavy loss of activity during a protracted use at elevated temperatures. An invention which consists in using zirconia as a thermally stable sintered article is disclosed, for example, in Japanese Patent Publication SHO 57(1982)-2,382. The thermally stable sintered article of zirconia is deficient in binding force with such catalyst components as platinum-family metal and ceria and liable to induce growth of particles of the active component and growth of crystals and, as a result, the produced catalyst suffers from low thermal stability and poor durability.
An object of this invention, therefore, is to provide a novel catalyst for purifying exhaust gas and a method for the production thereof.
Another object of this invention is to provide a catalyst for purifying exhaust gas which retains outstanding durability when it is used under such harsh conditions as encountered in an oxidizing atmosphere at elevated temperatures and exhibits a high scrubbing ability at low temperatures to the harmful components contained in the exhaust gas.